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Horizontal & Vertical Saccades: Circuitry
Horizontal Saccades: Circuitry
SUPERIOR COLLICULUS
- The superior colliculus is commonly divided into a dorsal, visuosensory division and a ventral, motor division.
- Dorsal division receives an organized retinotopic map of the contralateral visual hemifield from retinal ganglion cells and also receives afferent input from visual cortical regions, as well, including the striate, extrastriate, and frontal cortices.
- Ventral division projects to the contralateral abducens nucleus.
ABDUCENS NUCLEI
Pons
- Span from the mid-pons to the low pons.
EXCITATORY BURST NEURONS
Pons
- Lie in the paramedian pontine reticular formation in the mid-pons, anterior to the superior aspect of the abducens nucleus.
INHIBITORY BURST NEURONS
Medulla
- Lie in the medullary reticular formation, within the rostral medulla, anterior to the plane of the abducens nucleus.
NEURAL INTEGRATOR
Medulla
- Lies along the dorsal tegmentum of the upper medulla, just anterior to the fourth ventricle.
OMNIPAUSE NEURONS
Pons
- Lie in midline in between the rootlets of the abducens nerves in the pontine tegmentum.
HORIZONTAL SACCADE CIRCUITRY
Brainstem
The frontal eye fields (think: post. middle frontal gyrus*) directly excite contralateral excitatory burst neurons AND also send an indirect excitatory pathway that synapses within the superior colliculus, which, in turn, excites the contralateral excitatory burst neurons.
- The excitatory burst neurons excite the ipsilateral abducens nucleus, which stimulates the final common pathway for horizontal saccades.
- In addition to exciting the abducens nucleus, the excitatory burst neurons also excite the ipsilateral inhibitory burst neurons, which inhibit the contralateral abducens nucleus.
- For example, when the left abducens nucleus is stimulated, the right abducens nucleus is inactivated, which prevents both nuclei from firing at the same time.
- The omnipause cells tonically inhibit the excitatory burst neurons and also the inhibitory burst neurons.
- The excitatory burst neurons inhibit the omnipause neurons and inactivate their tonic suppression of the burst neurons.
- The neural integrator receives fibers from and projects fibers to a wide array of nuclei, including the excitatory and inhibitory burst neurons, to sustain gaze.
Details:
SUPRA-OCULAR COMMAND CENTERS FOR HORIZONTAL SACCADES
Comprises the...*
- Frontal Eye Fields (the frontal eye field, the supplementary eye field, and dorsolateral prefrontal cortex) (altogether, think posterior middle frontal gyrus)
- Superior colliculus
- Cerebellum
- Anterior Cingulate Cortex
- Basal Ganglia
- Frontal eye fields and superior colliculus drive the eyes to the contralateral side.
- Injury to these structures produces contralateral volitional horizontal gaze palsy.
EXCITATORY BURST NEURONS
Pons
- Lie within the nucleus reticularis pontis caudalis of the paramedian pontine reticular formation (PPRF).
- The nucleus reticularis pontis caudalis lies within the mid-pons, anterior to the superior aspect of the abducens nucleus.
- The excitatory burst neurons activate the ipsilateral abducens nucleus and the ipsilateral inhibitory burst neurons, which inhibit the contralateral abducens nucleus.
- Excitatory burst neurons also inhibit the omnipause neurons and innervate the neural integrator.
- Injury to the PPRF produces ipsilateral horizontal gaze palsy.
INHIBITORY BURST NEURONS
Medulla
- Lie within the nucleus paragigantocellularis dorsalis of the medullary reticular formation.
- The nucleus paragigantocellularis dorsalis lies within the rostral medulla, anterior to the plane of the abducens nucleus.
- The inhibitory burst neurons, most notably, suppress the contralateral abducens nucleus from firing, which prevents antagonist forces on the intended horizontal saccade movement.
- Injury to the inhibitory burst neurons is hypothesized to produce ocular flutter, high-frequency conjugate horizontal saccades without an intersaccadic interval.
NEURAL INTEGRATOR
Medulla
- Lies within the medial vestibular nucleus and the nucleus prepositus hypoglossi of the perihypoglossal complex, which lie along the dorsal tegmentum of the upper medulla just anterior to the fourth ventricle.
- The neural integrator produces gaze holding.
- Injury to the neural integrator causes a leaky integrator, which means that the eyes do not remain in the intended direction of gaze but instead drift back to center, prompting a corrective saccade.
OMNIPAUSE NEURONS
Pons
- Lie within the nucleus raphe interpositus, which sits in midline in between the rootlets of the abducens nerves in the pontine tegmentum.
- Omnipause cells tonically suppress the excitatory and inhibitory burst neurons except immediately before and during saccadic eye movements.
- Injury causes a slowing of saccades.
Vertical Saccades:
ROSTRAL INTERSTITIAL NUCLEI OF THE MEDIAL LONGITUDINAL FASCICULUS (RIMLF)
Midbrain-diencephalic junction
- Lie at the midbrain-diencephalic junction, in the plane of the mammillary bodies.
- Comprises the excitatory burst neurons for vertical and torsional saccades.
- Acts in similar fashion to the nucleus reticularis pontis caudalis of the PPRF for horizontal saccades.
INTERSTITIAL NUCLEI OF CAJAL (INC)
Midbrain-diencephalic junction
- Lie just beneath the bilateral riMLF, still in the midbrain-diencephalic junction, near midline in the dorsal tegmentum.
- Acts as the neural integrator for vertical and torsional saccades, similar to the medial vestibular nucleus and nucleus prepositus hypoglossi for horizontal saccades.
- Certain texts describe the INC as containing the inhibitory burst neurons for vertical and torsional saccades whereas others describe the riMLF as containing them.
- For comparison, the nucleus paragigantocellularis dorsalis of the medullary reticular formation contains the inhibitory burst neurons for horizontal saccades.
OCULOMOTOR COMPLEX
Midbrain
- Lies beneath the bilateral INCs within the dorsal tegmentum just anterior to the periaqueductal gray area.
- Straddles the midline of the mid- to upper midbrain.
TROCHLEAR NUCLEI
Midbrain
- Lie beneath the oculomotor complex in the lower midbrain (in the plane of the inferior colliculi) in the dorsal tegmentum near midline.
NUCLEUS RAPHE INTERPOSITUS
Pons
- Lies in midline in the pontine tegmentum in between the rootlets of the abducens nerves.
- Contains the omnipause neurons for vertical and torsional saccades just as it does for horizontal saccades.
KEY POINTS
- 1) The riMLF produces bilateral projections for upward gaze but only ipsilateral projections for downward gaze.
- Thus, a unilateral riMLF lesion will cause isolated downward gaze palsy (observed as slowing of downward gaze), but redundant innervation for upward gaze preserves the function and speed of upward gaze.
- 2) Laterality of riMLF. Innervation for torsional saccades is as follows: from the subject's POV, the right riMLF produces clockwise torsional rotation of the eyes and the left riMLF produces counterclockwise torsional rotation of the eyes.